Paper coating compositions are used in the paper industry to provide various physical properties and desired appearances to paper. Generally, paper coating compositions are composed of an aqueous dispersion that include pigments (e.g., clay, calcium carbonate, and/or titanium dioxide) and binders, among other components.
Paper coating compositions can be applied to a continuous web of paper by high speed coating machines, such as blade coaters, air knife coaters, rod coaters, and roll coaters. It can be advantageous to use faster coaters to increase productivity and to use higher solids coating compositions to decrease drying costs and improve binder distribution. So, the flow properties of the paper coating composition are important with regard to the rheology and runability of the composition during the coating operation.
Coating compositions often include a thickener to control flow properties of the paper coating composition. The coating compositions can also include components such humectants, lubricants, defoamers, crosslinkers, and optical brightening agents (OBAs). While OBAs alone work well in the coating composition, the use of polyvinyl alcohol (PVOH) in the coating composition boosts their performance. The exact mechanism of how and why this occurs is not fully understood, but it is believed that PVOH forms a complex with the OBA, thereby fixing the OBA in its active conformation (P. Hentzschel, Polyvinyl Alcohol, in: Pigment Coating and Surface Sizing, Fapet Oy, Helsinki, 2000).
PVOH can be supplied to papermakers in a variety of forms. These forms can include a dry resin that is brought into an aqueous solution at about 20 percent by weight to about 30 percent by weight solids by the customer. The advantage with this approach is that less costly grades of PVOH can be used. Another form of PVOH is one that has been pre-dissolved by the manufacture and delivered to the customer at about 15 percent by weight to about 25 percent by weight solids. This is costly to the papermaker as it involves shipping mostly water. Finally, PVOH can be supplied as dry, fine particle size grades that are added dry to the paper coating composition. This latter product is a relatively expensive ground material, but is convenient if on-site suspension is not possible.
Regardless of the form, the percentage of PVOH solids in the coating composition is typically limited by the viscosity required for the paper coating process. In addition, the PVOH can oftentimes interact strongly with the binder and pigments in the paper coating. The result is a paper coating composition having poor rheology and a negative impact on the paper coating process. As a result, a suitable alternative to the use of PVOH as a carrier for OBAs in paper coating is desired.